Measuring total corneal power before and after laser in situ keratomileusis with high-speed optical coherence tomography

Abstract

PURPOSE: To measure total corneal power using optical coherence tomography (OCT). SETTING:Refractive surgery practices at 2 academic eye centers in Cleveland, Ohio, and Los Angeles, California, USA. METHODS:Thirty-two eyes of 17 patients having myopic laser in situ keratomileusis (LASIK) were enrolled in a prospective observational study. Manifest refraction, OCT, and Placido ring corneal topography with the Atlas 995 (Carl Zeiss Meditec, Inc.) were performed preoperatively and 3 months after laser in situ keratomileusis (LASIK). A high-speed (2000 axial scans/second) corneal and anterior segment OCT prototype was used. The total corneal power was calculated by summation of the anterior and posterior surface powers, and the value was compared with that determined by simulated keratometry. Two methods of measuring total corneal power were tested: the direct method, which used OCT to measure both corneal surfaces directly, and the hybrid method, which combined OCT with anterior corneal topography. RESULTS:The repeatability (pooled standard deviation) of measuring total corneal power using the hybrid method was 3 times better than that using the direct method. It was 0.23 diopter (D) before LASIK and 0.26 D after LASIK. Preoperative total power was 1.13 D (2.6%) lower than the simulated keratometry. Compared to the LASIK-induced change in spherical equivalent refraction, the change in total corneal power was equivalent, while the change in simulated keratometry power was significantly smaller (-18.8%) (P<.001). CONCLUSIONS:Keratometry using the traditional index of 1.3375 overestimated the total power in preoperative corneas and underestimated LASIK-induced refractive change. Measuring both corneal surfaces using a combination of OCT and Placido ring topography provided a better measure of total corneal power that closely tracked the refractive change in post-LASIK eyes.Corneal power is determined by the 2 refractive surfaces at the anterior and posterior boundaries. It is commonly measured by manual keratometry or simulated keratometry output from computerized corneal topography systems. Because keratometry measures only the central 3.0 mm area 1 on the anterior corneal surface, 2 assumptions are needed to extrapolate corneal power. First, the corneal surface has uniform curvature and power. Second, the posterior/anterior curvature ratio maintains a fixed value across the normal population. This second assumption is implicit in the keratometric index of 1.3375 that is traditionally used to

Ray-tracing intraocular lens power calculation using anterior segment optical coherence tomography measurements

“…Anterior segment optical coherence tomography (AS-OCT) can measure the entire shape of both corneal surfaces with a nearinfrared light 15 without image degradation when there is corneal opacity. 16 Recent studies 17,18 found that the topography and CCT data obtained using AS-OCT agree well with conventional measurements.…”

A new method to accurately assess post-laser-surgery refractive changes with the intrinsic corneal power changes

“…In a preliminary study, Savini, et al [6] reported that the Total Refractive Power (the power determined by corneal ray tracing, in which Snell's law and specific indices of air, the cornea and the aqueous are used) on the Pentacam HR seems to be an accurate method to evaluate surgically induced refractive changes. Using high-speed optical coherence tomography (OCT), Tang [1] found that the difference between total power for 3.0 mm and 4.0 mm zones (calculated by the radius of the corneal anterior surface and posterior surface with the real corneal index of 1.376) changes, and LASIK-induced refraction changes is 0.16 D with a correlation coefficient of 0.94, while simulated keratometry underestimated refraction the change by 0.60 D, 18.9% by linear regression. Therefore, these investigators concluded that measuring both anterior and posterior corneal surfaces with a combination of OCT and Placido ring topography provided a better measure of total corneal power that closely tracked the surgically induced refractive changes.…”

“…Corneal changes given by corneal topographies are generally believed to underestimate the actual surgically induced refractive changes [1]. Several explanations, such as the invalid keratometric refraction index of 1.3375 [1,2], the change in posterior surface [3,4] and corneal index change after refractive surgery [5] were proposed to explain this phenomenon. Subsequently, different methods [6][7][8] to accurately assess the refractive changes have been proposed.…”

Anterior Eye Imaging with Optical Coherence Tomography

“…In a study of 32 eyes from 17 LASIK patients, we evaluated the repeatability of OCT-based corneal power measurement by pooled standard deviation [61]. The repeatability is 0.79D for the anterior surface and 0.10D for the posterior surface.…”

“…At 2,000 axial scan per second, eye movement during OCT scans still produces noticeable error in corneal power measurement. Nevertheless, if we combine the OCT-measured pachymetry map and anterior corneal topography, the resulting total corneal power can closely track the refractive change after LASIK [61].…”